The controlling parameter in MOSFET is

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$V_{GS}$

Explanation: The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled device where the gate-to-source voltage ($V_{GS}$) regulates the flow of current between the drain and source. Unlike BJTs which are current-controlled, the MOSFET's input impedance is extremely high, meaning the gate draws negligible current.

Working Principle: When $V_{GS}$ exceeds the threshold voltage ($V_{th}$), an inversion layer (channel) is formed between the source and drain regions. By modulating the gate voltage, the width and conductivity of this channel change, thereby controlling the drain current ($I_D$). The gate is isolated from the channel by a thin $SiO_2$ layer, preventing current flow into the gate terminal.

Key Formulas: $I_D = \frac{1}{2} \mu_n C_{ox} \frac{W}{L} (V_{GS} - V_{th})^2$ — Drain current in saturation region; $I_G \approx 0$ — Ideal gate current for MOSFET

Key Points: MOSFET is a unipolar device.; It is a majority carrier device.; The gate electrode is capacitively coupled to the channel.; High input impedance makes it ideal for high-frequency switching.

Advantages: Extremely low gate drive power; High switching speed

Disadvantages: Susceptible to electrostatic discharge (ESD) damage; High on-state resistance at higher voltage ratings

Applications: Switch-mode power supplies (SMPS); Digital logic circuits and microprocessors

Additional Info: The $V_{DS}$ (drain-to-source voltage) determines the operation region (ohmic vs. saturation) but $V_{GS}$ remains the primary controlling parameter.. Option B ($I_G$) is negligible due to the insulating oxide layer.. Option D ($I_S$) is the result of the drain current flow, not the control variable.

Answer: C is correct — The MOSFET is a voltage-controlled device where the channel conductivity is modulated by the gate-to-source voltage $V_{GS}$.

Exam Tip: Always remember: BJT = Current Controlled (base current $I_B$); MOSFET = Voltage Controlled (gate voltage $V_{GS}$).

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